1. Field of the Invention
The present invention relates to a method of manufacturing a sintered machine part using a powder metallurgical method, and more particularly, to a method of manufacturing a machine part such as a synchronizer hub for a motorcar which requires a highly accurate size.
2. Description of the Prior Art
A transmission part for a motorcar such as a synchronizer hub operates under a bending force and a tensile force due to its sliding motion with respect to driving shafts, a sleeve and a ring of a synchronizer. For this reason, it is required to have a sufficient durability against recursive impacts such as those associated with gear shifting operations.
Such a synchronizer hub has been manufactured by method of powder metallurgy since more than twenty years ago, but a motorcar of those days had a relatively small engine power so that each machine part of the engine were loaded only lightly. For this reason, an ordinary sintered steel composed of iron, copper and carbon has been satisfactory as a material for the synchronizer hub.
However, as a result of recent regulation on the exhaust gas, and new technological developments after the oil shock crisis, especially those related to a high-power and high performance engine, the materials used for the synchronizer hub in the past had become insufficient. For this reason, the material for the synchronizer hub has been changed to a high alloy sintered steel having additive ingredients such as chromium and nickel, whose strength and wear and abrasion resistance are further improved by various heat treatments such as carburizing and induction hardening.
Yet, more recent trend requires such a high grade motorcar which is excellent not only in terms of the engine performance, but also in terms of comfortableness such as a smallness of noises and vibrations transmitted to a driver's seat during the running and a smoothness of the gear shifting operation.
Now, there are many factors related to the realization of such a high grade motorcar. As for the transmission parts, each part is required to be smoothly finished, without distortion, and capable of making a tight engagement with a partner part while being smoothly slidable each other. These in turn necessitate far more stringent precision for the sizes of the parts and accurate manufacturing of such parts, in a case of the synchronizer hub.
Here, the problem arises in relation to the heat treatments after the sintering. Namely, the conventional sintered alloy cannot achieves satisfactory mechanical properties without the heat treatments, while on the other hand, the heat treatments cause the distortion on the manufactured parts, so that it has been difficult to preserve the precision of the sizes of the parts.